This invention relates to the display of video signals on an output device. More specifically, the invention relates to a system and apparatus for enhancing the quality of output signals that have been converted from a lower resolution format.
Television images are created when an electron gun inside a cathode ray tube delivers a series of horizontal lines to a phosphor. The glow from the phosphor is reflected on to the monitor to create the image. In the United States, television broadcast standards are currently set by the National Television Standards Committee (NTSC). In accordance with these standards, 480 lines are displayed on the screen at a time, and the image on the screen is displayed 60 times per second. While these lines are displayed together, it is important to note that all lines are not always re-drawn by the electron gun at the same time. In addition to setting the number of lines that are displayed on the screen, the NTSC standard provides for an xe2x80x9cinterlacedxe2x80x9d display system. This means that the 480 lines are drawn in two groups of 240 lines each, with the odd numbered lines being drawn during one pass, and the even numbered lines being drawn during another. In other words, each group of lines is displayed 30 times per second. The system described above is typically referred to as a xe2x80x9c480ixe2x80x9d system.
The NTSC standard provides for an analog signal. However, there is a significant effort underway to deliver digital television (DTV) signals to households, and it is almost a certainty that DTV broadcasts will replace analog television broadcasts in the near future. There are numerous DTV formats. Some of these use an interlaced system such as that described above, while other use a xe2x80x9cprogressivexe2x80x9d format. In a progressive system, all lines of resolution are re-drawn during each pass. Thus, while a 480p system will have the same number of horizontal lines as a 480i system, the 480p image will appear to be much more smooth because the lines in the image will be re-drawn twice as fast. DTV standards are being set by the Advanced Television Systems Committee (ATSC), an international organization. The ATSC has assigned 18 different DTV formats, which provide for different numbers of lines, as well as use of both the progressive and interlaced formats. Currently, the most common formats are 480p, 720p and 1080i.
These developments have created a need for television systems that can receive and display television broadcast signals that are presented in different formats. One way to accomplish such a task is to include a device in the television set that can receive signals in an incompatible format, and convert it to the format that the television set can use. Conversions between formats often result in the loss of data, which deteriorates the quality of the image that is displayed. For example, upconversionxe2x80x94conversion from a low resolution format to a higher resolution formatxe2x80x94typically requires interpolating the originally provided broadcast signal. For example, in the near future, many consumers will have televisions that are capable of displaying High Definition (HD) broadcast signals. However, televisions signals are currently broadcast in the lower resolution Standard Definition (SD) format. It will be some time before television stations convert their broadcasts to the HD format. Thus, it is necessary to include a device in a HD television set that can receive SD signals and convert them to the HD format.
The upconversion process typically requires the use of an interpolation filter, to remove the high frequency images that are contained in upsampled pictures. More specifically, during the upconversion process some pixel values will be set equal to zero. If these zero value pixels are displayed as is, they will appear as black spots on the television screen. Instead, an interpolation filter is applied to the upsampled image to reset them to non-zero values, and thereby generate a more visually appealing reproduction of the picture that has been converted. As a practical matter, the filter that is applied to the upconverted image will not be idealxe2x80x94not all of the high frequency images will be removed, and thus, some very weak high frequency energy will remain in the upconverted picture. To human vision, the high frequency data in an upconverted image is usually perceived as blurring. One way to eliminate this blurring is to sharpen the edges of the picture and to enhance the contrast of its texture areas. This edge sharpening and contrast enhancement creates new high frequency components in the picture, and bursts the existing mid level frequencies. While it is typically not possible to recreate all of the details that are lost during low resolution video production (e.g. SD video recording), these processes help to restore at least some of the quality of the original picture. However, a need remains for improved methods and systems for processing images that have been converted from another format, particularly those that minimize processing and computation time, in order to enhance their quality.
In accordance with an embodiment of the invention, a method of enhancing the resolution of a video signal includes acquiring a relatively low resolution video signal, such as a Standard Definition Television signal. A peaking function is applied to the low resolution signal and the peaked signal is analyzed to identify pixels that may be edges. The peaked signal is then xe2x80x9cupconvertedxe2x80x9dxe2x80x94its resolution is increased to that of the desired output, such as a High Definition Television signal. An edge detection function is then applied to the higher resolution video signal, specifically at the pixels that correspond to those that were previously identified as possibly being edges. Edge linking is applied to the detected edges, a luminance transition improvement function is applied to the detected edges and the processed high resolution signal is output to a high resolution display.
In accordance with the invention, application of the peaking function increases image quality by increasing the perceived contrast of the original image. That is, it will appear to the human eye that the contrast of the image has been increased, particularly in its textured regions. The peaking function is typically applied prior to upconversion. The edge detection function is then applied after upconversion to accurately determine which of the upconverted pixels are edges. Functions that are provided in accordance with the invention produce estimated values for characteristics of the image, which reduces computation time while maintaining image quality. Detected edges in the image are linked and the image is sharpened in the horizontal and vertical directions. According to the invention, various parameters can be altered to scale the complexity of the processing operations, and control the level of resolution enhancement that takes place.
Other embodiments of the present invention and features thereof will become apparent from the following detailed description, considered in conjunction with the accompanying drawing figures.